Transdermally absorbable preparations have recently come into notice as the preparations for administering drugs to living bodies from the viewpoint of handling facility and dosage control. Dosage forms of the transdermally absorbable preparations mainly include transdermally absorbable preparations of reservoir type and matrix type.
The transdermally absorbable preparations of reservoir type are those which are provided with an adhesive layer, a release controlling membrane and a drug containing layer in order from the side of skin. For instance, it has been described in Japanese Patent Laid-Open Publication No. 2003-63954 that the releasability of a drug from an adhesive layer is improved by uniformly dispersing a particulate inorganic salt or a water soluble polymer into the adhesive layer of a transdermally absorbable preparation of reservoir type. However, it is necessary for the transdermally absorbable preparations of reservoir type to optimize the constitution of the adhesive layer by taking account of the nature and releasing period of the drug, which takes time and labor. Moreover, transdermally absorbable preparations of reservoir type have the problem that constituents including a drug are transferred to the adhesive layer with the passage of time resulting in the alteration of the physical properties of the adhesive layer and thus decreasing remarkably both the adherence ability onto skin and the drug releasing ability.
Further, transdermally absorbable preparations of matrix type are known as the preparations in which the adhesive layer contains a drug. Transdermally absorbable preparations of matrix type can be prepared easily as compared with those of reservoir type. However, it has been described in U.S. Pat. No. 6,465,004 that in transdermally absorbable preparations of matrix type the drug tends to be crystallized from the adhesive layer during its shelf life thus leading to the adherence ability onto skin and the decrease in the drug releasing ability. Moreover, in transdermally absorbable preparations of matrix type, the amounts of the constituents such as the drug and the absorption promoting agent are limited for maintaining their adherence ability onto skin.
Furthermore, exercise and bathing are generally limited during the actual use of a transdermally absorbable preparation in order to avoid its peeling off from the skin. However, it is difficult to avoid the decrease with the passage of time in the drug releasing ability and adhesion ability of the preparation in spite of such limitations. Thus, it has still been needed to create a transdermally absorbable preparation which enables the efficient, stable and long-term administration of a drug to a living body.